The use of centrifuges is common in hematology for the preparation and spinning of plasma and serum blood samples. Centrifugation causes the incrementally more dense red blood cells to separate out from the remaining plasma. This is useful for determining the packed cell volume hematocrit value of whole blood, as well as for determining coagulation related and other hematological parameters.
A conventional hematology associated centrifuge comprises a rotor head mounted on a vertical drive shaft, extending upwardly from an electrical motor assembly housed within a base. The head typically takes the form of a body of rotation having a centrally disposed hub portion and an annular portion. The hub portion serves to mount the head for rotation on the drive shaft. The annular portion defines a plurality of cavities or sockets that, either directly or indirectly through the use of fixed or "swinging bucket" adapter sleeves, provide open-topped receptacles for holding a corresponding plurality of sample tubes during spinning. For sake of balance, the cavities are equiangularly spaced about the rotational axis of the head. In order to facilitate insertion and removal, cavity lengths are preferably matched to tube standard body lengths so that the necks of the tubes protrude out from the cavity openings when the tubes are fully inserted.
Following traditional teachings, prior art centrifuge devices have cavities oriented either horizontally or at angles of about 45.degree. with respect to the rotational axis. It has, however, been recognized in connection with the centrifugation of microhematocrit capillary tubes for determination of packed cell volume, that the rate of migration of red blood cells from plasma can be increased by orienting the tubes almost vertically, at angles of about 20.degree. to the rotor axis (70.degree. to the horizontal plane of the hub portion). The benefits to be obtained from such steep angling are discussed in Brimhall et al. U.S. Pat. No. 4,738,655. The extension of such steep angling to larger tubes, though, is subject to several disadvantages. The tubes are subject to increased risk of breakage, especially at the cavity opening/tube neck interface, due to greater cross-axial forces applied to the tubes. There is also an increased risk of stopper dislodgement due to greater cross-axial forces applied to the stoppers. This results in an attendant increase in the risk of contamination due to biohazardous material.